Distribution of laser shot noise energy delivered to a levitated nanoparticle

Abstract

This paper quantifies the rate at which laser shot noise energy is delivered to a nanoparticle for the various scenarios commonly encountered in levitated optomechanics. While previous articles have the same form and dependencies, the proportionality constants often differ in the literature. This paper resolves these discrepancies. The rate at which energy is delivered to an optically trapped particle's respective degrees of freedom depends on the radiation pattern of scattered light as well as the direction of laser propagation. For a traveling plane wave with linearly polarized light, in the Rayleigh regime this leads the translational shot noise heating rate to be proportional to $1/10$ of the total rate in the laser polarization direction, $7/10$ in the laser propagation direction, and $2/10$ in the direction perpendicular to both. Analytical expressions for the shot noise heating rate are provided in the Rayleigh limit as well as numerical calculations for particles in the Mie regime for silica and diamond. For completeness, numerical calculations of the shot noise heating for silica Mie particles at the focus of a strongly focused laser beam are calculated for varying numerical aperture and common laser wavelengths. Both numerical calculations show that the Rayleigh expression generally gives an overestimate of the shot noise heating especially for larger radii, but is still a good approximation even for incident focal fields. The exception to the relative decrease is when a Mie resonance is reached which was found for diamond. Lastly, Rayleigh expressions for the rotational shot noise heating for a symmetric top-like particle for linear, elliptically, and unpolarized light are also provided.